[doc] Fix info on the shape of pivots in `torch.lu` + more info on what and how they encode permutations. (#46844)
Summary:
As per title.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46844
Reviewed By: VitalyFedyunin
Differential Revision: D24595538
Pulled By: ezyang
fbshipit-source-id: 1bb9c0310170124c3b6e33bd26ce38c22b36e926
